1. Field of the Invention
This invention relates to fluid flow rate controls for hydraulic pumps, especially for automotive power steering pumps. The invention pertains to a device for enhancing the sensitivity of such a control to conditions of high viscosity so that an effect of cold start pump cavitation is eliminated.
2. Description of the Prior Art
At temperature near -40.degree. F., viscosity or resistance to flow, of fluid used in automotive power steering systems increases by about 8000 times its viscosity at 275.degree. F. At low temperature, the fluid flows like thick, heavy syrup at room temperature.
Conventionally, power steering systems have a reservoir located remotely from the hydraulic pump that pressurizes the system. The remote reservoir allows its placement in a relatively uncongested region in comparison to the region surrounding the pump and drive belt sheave, by which the pump is driven from an engine. A pressure drop of 5-7 psi occurs at low temperature in a tube connecting the remote reservoir to the pump inlet. Another pressure drop of about the same magnitude is present within the pump between its inlet and the pumping chamber. These pressure drops cause an extremely low pressure, about 1 psi., in the pump chamber at low temperature.
When the engine is started in cold weather, pump speed immediately rises, but viscosity is too high to permit sufficient fluid from the remote reservoir to enter and to fill the pumping chamber. This cavitates the pump and causes an offensive high frequency scream lasting several seconds as fluid pressure in the steering gear supplied from the pump cycles rapidly between zero pressure to 100 psi. The cyclic nature of the pressure variation is a consequence of successive short periods of slug-like flow through the pump, when a pumping chamber is at least partially filled with fluid, followed by a short period when the pumping chambers are substantially fully vacant.
The characteristic noise is objectionable and evidences a brief period during which the system or load is only partially pressurized. As flow rate increases, fluid temperature rises rapidly to a temperature where cavitation ceases, system becomes fully pressurized, noise disappears, and function is normal.
To overcome the cold start difficulties, it is conventional practice to increase the size of hoses connecting the pump to the steering gear and the reservoir to the pump inlet in order to enhance flow. Hydraulic fluid, whose viscosity increases only about 4000 times between 275.degree. F. and -40.degree. F. is used at a substantial increase in cost over fluid having the usual viscosity increase over this temperature range.
Various techniques for controlling operation of the flow control valve have been developed. For example, U.S. Pat. No. 4,289,454 describes a vane pump having two outlet ports, one port being closed after the flow rate exceeds a predetermined magnitude due to an increase in speed of the rotor. The excess fluid normally passing through one of the outlet ports is returned to the pump inlet to increase the fluid flow rate to the steering gear during high speed conditions.
U.S. Pat. No. 4,470,762 describes a pump having a control that bypasses flow from the pump between a cam ring and thrust plate. A spring opens the bypass passage and a pressure plate closes the bypass passage when system pressure rises. The pump control described in U.S. Pat. No. 4,470,764 includes a spring operating on a valve spool to open bypass flow and biased by system pressure to reduce bypass flow. In the vane pump of U.S. Pat. No. 4,470,765, output flow is partially bypassed through a flow control valve. The valve is operated by system pressure to close bypass passages as system pressure rises, thereby increasing flow to the power steering system.
More recently, power steering systems include electronically variable orifices that are opened and closed in response to vehicle speed and steering wheel speed so that the flow rate to the steering gear from the pump outlet is high when the required steering assist is high, particularly at low vehicle speed, and is low when the required steering assist is low, particularly at high vehicle speed and low steering wheel speed. An example of a power steering system controlled in this way is described in U.S. Pat. No. 4,473,128 in which a bypass valve directs a portion of the fluid flow from the pump from the steering gear in response to vehicle speed and angular velocity of the steering wheel. The position of the bypass valve is controlled by a solenoid, energized and deenergized on the basis of control algorithms executed by a microprocessor. The flow control valve described in U.S. Pat. No. 4,691,619 is also operated by a solenoid, which is energized and deenergized in response to vehicle speed. A pressure modulated slide valve is hydraulically piloted by a solenoid-operated valve. Fluid flow to the steering gear is controlled entirely hydraulically in response to vehicle speed and demand requirements represented by the steering gear input.
U.S. Pat. No. 4,485,883 describes a power steering system having a bypass valve controlling the flow rate of fluid directed from the pump outlet to the pump inlet and a constant flow valve for regulating the flow of bypass fluid. This control system reduced the flow rate to the steering gear during steering maneuvers at high speed and increases the flow rate at low speed and during parking maneuvers.
A similar object is realized with the power steering systems described in U.S. Pat. Nos. 4,561,561; 4,570,735. A vehicle speed sensitive valve operates to deactivate a conventional flow control bypass valve by eliminating differential force on the flow control valve at speeds greater than a predetermined value. U.S. Pat. No. 4,714,413 describes a power steering system of this type. Another control system of this type employing a solenoid-operated vehicle speed sensitive valve in combination with a conventional flow control bypass valve is described in U.S. Pat. No. 4,609,331.